SPP1 expression in the mouse uterus and placenta: implications for implantation†

Area of Science:

• Reproductive biology
• Developmental biology
• Cellular communication

Background:

• Secreted phosphoprotein 1 (SPP1), also known as osteopontin, mediates cell interactions.
• SPP1 is linked to pregnancy, but its precise roles in mouse implantation and placentation require clarification.
• Existing in vivo data on SPP1's localization and function during pregnancy is incomplete and contradictory.

Purpose of the Study:

• To precisely localize SPP1 mRNA and protein within the mouse endometrium and placenta throughout gestation.
• To investigate the functional role of SPP1 in blastocyst implantation using a delayed implantation model.
• To elucidate the specific contributions of SPP1 to uterine closure, blastocyst attachment, invasion, and placental development.

Main Methods:

• Localization of Spp1 mRNA and protein using in situ hybridization and immunohistochemistry in mouse uteri and placentas.
• Delayed implantation model utilizing mouse blastocysts to study SPP1's role in the implantation chamber.
• Analysis of SPP1 expression in endometrial epithelia, uterine natural killer (uNK) cells, trophoblast cells, and parietal endoderm.

Main Results:

• SPP1 mRNA and protein were found in endometrial luminal epithelia (LE) at interimplantation sites throughout gestation.
• SPP1 localized to uNK cells in the decidua and intermittently to LE cells and inner cell mass within the implantation chamber.
• SPP1 protein was detected in intermittent trophoblast cells and parietal endoderm within the implantation chamber.

Conclusions:

• SPP1 is secreted by LE at interimplantation sites, facilitating uterine lumen closure to form the implantation chamber.
• SPP1 secreted by LE near attaching trophoblasts aids blastocyst attachment and invasion.
• SPP1 from uNK cells in the decidua promotes angiogenesis, supporting embryonic/fetal development via hemotrophic support.